Styrenic polymer foams are produced commercially by extrusion and foaming of molten polymers, or by expansion and molding of polymer particles. Blowing agents such as pentane are routinely used to expand the polymers to produce foams. Increasingly tough legislation limits allowable emissions of volatile organic compounds (VOC), so foam manufacturers need ways to foam polymer particles with reduced levels of blowing agents.
Polystyrene particles having improved expandability can be prepared by polymerizing styrene in aqueous suspension in the presence of chain-transfer agents or styrene oligomers. The method of Hahn et al. (U.S. Pat. Nos. 4,525,484, 4,520,135, and 4,513,105) illustrates this approach. A chain-transfer agent, such as n-dodecyl mercaptan, is introduced during styrene polymerization. The polymer product has a reduced proportion of very high molecular weight polystyrene, and has improved expandability compared with products made in the absence of the chain-transfer agent.
Additional routes to styrenic polymer particles with improved expandability are needed. A desirable approach would eliminate the need for introducing a chain-transfer agent at a particular stage of the polymerization. Ideally, the method would give high-quality styrenic polymer particles having improved expandability at little or no added cost. In addition, the heat sensitivity of the polymer particles, i.e., the tendency of the particles to shrink during pre-expansion, would be minimized.